It was a long time ago to use a golf ball with circular dimples on its outer surface which is divided into the faces of an spherical polyhedron. The golf ball with dimples on its outer surface has a merit of long distance flight by providing symmetrical balance on the ball between right and left side and distributing air resistance evenly all over the ball surface. The compositions in dividing the sphere's surface of golf ball that are widely used at present include spherical icosahedron, spherical icosidodecahedron, spherical octahedron, spherical hexaoctahedron, spherical dodecahedron, or further divide into the faces of smaller polyhedron and the like. But, in reality, the compositions in dividing sphere's surface of aforementioned can be superimposed one another in a same sized sphere, therefore all of them may be considered as a same divisional composition in a broad sense. If circular dimples are arranged on the basis of above mentioned composition, the flying characteristic of a ball varies with both the area ratio and the volume ratio of dimples occupying the ball surface. However, it has been found that the balls which are manufactured with same materials, composite and a same production method, those balls with a difference of less than approximately 5% in the area ratio or volume ratio of dimples on the ball's surface achieved similar flying characteristic and similar carry distance although the dimples are arranged by several different compositions in dividing sphere's surface.
Hereupon, this inventor have closely examined flying characteristics of different golf balls, and eventually invented a ball that secures flying stability and longer carry distance based on the following mechanism. A golfer hits a golf ball, strong repulsive elasticity is generated on the ball by the power applied from the head of a golf club, at the same time back spin is generated by the loft angle of a golf club. If the club is driver, the impacted ball as explained above will fly away at an initial velocity of approx. 190.about.300 Km/Hr. and also be given back spin of approx. 2200.about.4500 R.P.M. at an initial state. At this moment, The dimples accelerates the transition of turbulent flow around the boundary layer of a rotating ball in flight through the high speed air stream, fluid(air) particles around the boundary layer get mixed and tangled mutually at the front part of the ball and it becomes difficult to be separated since energe is provided from outside of the layer, and consequently separation point moves backward and the width of separation region gets narrow, that the coefficient of drag is reduced. In the meantime, air pressure will increase beneath the ball rotating reversely whereas it decrease above the ball, as a result, aerodynamic lift equivalent to about 4.about.5 times of gravity is generated due to the Bernoulli effect, and it results to extend the carry distance of a ball. Additionally, it lowers a coefficient of drag even at a low-speed area by reducing the Critical Reynold's number.
However, it is difficult to extend a carry distance of a golf ball with the aforementioned conventional dimple arrangement using only circular dimples since the speed and rotation strength of the ball does not remain as initial state of hitting, they rapidly reduce from the peak of a ballistic trajectory to the landing point, accordingly the critical Reynold's number will rapidly increase and a coefficient of drag as well. Simply changing the compositions of dimple arrangement in dividing sphere's surface of a golf ball with different kind of spherical polyhedron, that resulted in the same situation.
In general, increasing the diameter of a circular dimple lowers a coefficient of drag in a low-speed area, whereas it raises the coefficient of drag in a high-speed area, on the contrary, decreasing the diameter of a circular dimple lowers a coefficient of drag in a high-speed area, but it tends to raise the coefficient of drag in a low-speed area. In consequence, proper combination of small diameter and large diameter dimples has been tried on the surface of a golf ball recently, however this also confronted with limitation.
To extend the carry distance of a golf ball, it is necessary to have excellent dimple arrangement that allows to minimize air resistance at both high-speed and low-speed areas, but there was no way to achieve everything in reality.
Meanwhile, in case of volume ratio of dimples on the surface of a golf ball having circular dimples only, has correlation with the area ratio of dimples on it's surface due to the definite size of a ball, it is impossible to make cavities for cover mold allowing to freely change the volume of dimples by ignoring the diameter of dimples and volume ratio to obtain fundamental lift. In other words, if the area ratio of dimples corresponds to about 75.about.84% of the total surface area of a golf ball having circular dimples only, the total volume of dimples on its surface will be around 350.about.500 mm.sup.3. A volume ratio for obtaining fundamental aerodynamic lift becomes proportional to the diameter of dimples, that is, to increase the diameter of dimples results in a large volume of dimples and to decrease the diameter of dimples results in a small volume of dimples. Drag of any substance is a combination of pressure drag and friction drag. The strength of pressure drag is affected by the shape of the substance and the stream direction against it, whereas the strength of friction drag varies with the shearing strength caused by the viscosity of fluid flowing the surface of the substance and the roughness of the surface of the substance. Also a coefficient of drag varies with the Reynold's number.
Therefore, there is no problem in terms of a carry distance at a high-speed area from the hitting point to the peak of a ballistic trajectory, because a coefficient of drag diminishes as the Reynold's number grows in that area. The problem raises in a low-speed area from the peak of the ballistic trajectory to the landing point. Therefore, to extend the carry distance of a golf ball, it is desirable to decrease a coefficient of drag, particularly in a low-speed area. However, simply increasing the size of dimples cannot extend the carry distance because it increases volume ratio of dimples, and in turn a coefficient of drag in a high-speed area will increase. As an increase in a coefficient of pressure drag in a high-speed area is caused by high volume ratio of large dimples, that could not diminish a total drag.